From the Super Mario Wiki, the Mario encyclopedia
The Boo-ray Machine is a machine in Mario & Luigi: Bowser's Inside Story and Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey that Yikk built before his death. The name is a play on "x-ray machine" and may also be a play on Blu-ray, the name for a type of disk-based media format.
Yikk explains that the light makes the insides of one's body visible, and that it shows Boo transparency waves, giving off light and other special properties. Once Bowser activates the machine (by punching the button in the original game, or standing on the platform in the remake), Mario and Luigi, who are inside of Bowser to pursue Durmite, are able to access more of the Energy Hold, where they follow after Durmite. If Bowser keeps the Boo-ray Machine activated, Mario and Luigi may see hidden blocks somewhere. If Bowser deactivates it, several Boo-like blocks may appear as platforms.
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